Counter electrode for dry disk type rectifiers



United States Patent COUNTER ELECTRODE FOR DRY DISK TYPE RECTIFIERS Hermann K. Strosche and Otto J. Klein, Nurnberg, Germany, assignors to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware No Drawing. Application December 10, 1952,

Serial No. 325,212

Claims priority, application Germany December 20, 1951 7 Claims. (Cl. 317-241) This invention relates to new and useful improvements in the manufacture of selenium rectifiers, and particularly to the composition and application of counter electrodes.

As is well known, selenium rectifiers are built up of layers; a metal base plate, followed by one or more layers including a layer of selenium and a counter electrode. The counter electrode may be an alloy of various metals, e. g. cadmium, tin, lead, and bismuth, and it may be sprayed or evaporated on the selenium layer, a thin intermediate layer being preferably interposed between the two. The intermediate layer may effect an increase of 25 in the current flow and is customarily selected from the metals belonging to the secondary group of the fourth and fifth groups of the periodic system of elements, e. g. bismuth. However, the intermediate layer retards and may even make impossiblethe customary electric forming process.

According to the present invention, some metal such as thallium, gallium, indium or silver is added to the intermediate layer to aid in the formation of the blocking layer. If 50% of the intermediate layer consists of such additive then forming will proceed satisfactorily, the transfer resistance will be decreased and the current flow increased.

The alloy or mixture of intermediate layer metal and additive metal may be applied to the selenium by evaporation in a vacuum, insuring the formation of a uniform, thin deposit. A mixture or alloy of bismuth and thallium has proved to be very efiective. Since these metals have almost the same melting point they may be deposited in the desired ratio without fractionation. The intermediate layer is so chosen that 500 micrograms per square centimeter of metal, but preferably less than 100 micrograms per square centimeter is deposited on the selenium surface.

Preferably a separate evaporator is used for each metal. The two evaporators can be close together so that the two steam jets will mix near the evaporators. However, the evaporators or the selenium plates may be moved repeat edly at short intervals and thus vaporize the selenium with both metals. The ratio of the two components may be varied by varying the distance of the selenium surface from the two evaporators, by varying the vaporization period, by varying the speed of vaporization or the movement of the selenium surface, etc.

If the deposited layer is as thin as desired but lacks the required transverse conductivity, an additional layer may be applied, e. g. by spraying on it a spray-metal, i. e. an alloy having a low melting point. However, bismuth, lead, tin or cadmium, or an alloy of some or all said metals may also be used for this purpose.

According to the present invention, an alloy of tin and cadmium, having a melting point of about 178 C., or Woods metal can also be used to form the counter electrode by spraying it on top of the intermediate layer of bismuth and thallium. Such rectifiers could readily be formed and the current flow through them would be 25% higher than through rectifiers equipped with the customary counter electrode.

We claim:

1. A selenium rectifier having a counter electrode, a thin layer intermediate the selenium and the counter electrode consisting of a metal of the secondary group of the fourth and fifth groups of the periodic system of elements, and an additive selected from the group consisting of thallium, indium, and gallium.

2. The selenium rectifier according to claim 1, and in which the intermediate layer is -500 micrograms per square centimeter.

3. The selenium rectifier according to claim 1, and in which the additive constitutes up to 50% of the weight of the intermediate layer.

4. The selenium rectifier according to claim 1, and in which the counter electrode consists of tin and cadmium.

5. A selenium rectifier having a counter electrode, and a thin layer intermediate the selenium and the counter electrode consisting of bismuth and thallium.

6. The selenium rectifier according to claim 5, and in which the counter electrode is an alloy of tin and cadmium having a melting point of about 178 C.

7. The selenium rectifier according to claim 1, and in which the counter electrode consists of Woods metal.

References Cited in the file of this patent UNITED STATES PATENTS 2,279,746 Thompson et al. Apr. 14, 1942 2,496,432 Blackburn Feb. 7, 1950 2,510,322 Shearer June 6, 1950 2,602,763 Scaff et al. July 8, 1952 2,603,693 Kircher July 15, 1952 2,718,688 Waldkotter Sept. 27, 1955 FOREIGN PATENTS I 551,698 Great Britain Mar. 5, 1943 576,671 Great Britain Apr. 15, 1946 

1. A SELENIUM RECTIFIER HAVING A COUNTER ELECTRODE, A THIN LAYER INTERMEDIATE THE SELENIUM AND THE COUNTER ELECTRODE CONSISTING OF A METAL OF THE SECONDARY GROUP OF THE FOURTH AND FIFTH GROUPS OF THE PERIODIC SYSTEM OF ELEMENTS, AND AN ADDITIVE SELECTED FROM THE GROUP CONSISTING OF THALLIUM, INDIUM, AND GALLIUM. 